1. Field of the Invention
The present invention relates to a MOS transistor structure and, more particularly, to a MOS transistor structure and a method of forming the structure with vertically and horizontally-elongated metal contacts.
2. Description of the Related Art
A metal oxide semiconductor (MOS) transistor is a well-known semiconductor device which can be implemented as either an n-channel (NMOS) device or a p-channel (PMOS) device. A MOS transistor has spaced-apart source and drain regions, which are separated by a channel, and a gate that lies over the channel. The gate is insulated from the channel by a gate dielectric layer.
A metal-gate MOS transistor is a type of MOS transistor that utilizes a metal gate and a high-k gate dielectric layer. Current-generation methods, e.g., 20 nm and below, fabricate metal-gate MOS transistors by forming the source and drain regions, the channel, the gate dielectric layer, and the gate as elongated structures.
Metal-gate MOS transistors are connected to a metal interconnect structure that electrically connects the MOS transistors together to form an electrical circuit. The metal interconnect structure includes layers of metal traces that are electrically isolated from each other by layers of isolation material, and metal vias that extend through the layers of isolation material to electrically connect adjacent layers of metal traces.
The metal interconnect structure also includes metal contacts that extend through the bottom layer of isolation material to make electrical connections to the source and drain regions of the MOS transistors. The metal contacts are formed in metal contact openings that extend through the bottom layer of isolation material to expose the source and drain regions.
Conventionally, the metal contacts are fabricated by forming a patterned hard mask on the bottom layer of isolation material. The patterned hard mask has a number of square openings that lie over the source and drain regions. Once the patterned hard mask has been formed, the bottom isolation layer is etched.
The etch forms source metal contact openings that expose the source regions, and drain metal contact openings that expose the drain regions. The source metal contact openings and the drain metal contact openings are square as a result of the square openings in the patterned hard mask. The patterned hard mask is then removed.
After this, silicide layers are formed on the exposed source and drain regions, followed by the formation of metal contacts that lie in the source and drain metal contact openings, and touch the source and drain silicide layers and the bottom isolation layer. The metal contacts are square as a result of the square source and drain metal contact openings.